Multi-aquifer communicated stratum foundation pit deep and shallow well dewatering construction method

Abstract

The invention discloses a deep and shallow well dewatering construction method for a multi-aquifer communicated stratum foundation pit. In order to solve the problems of sedimentation outside a pit and wall deformation caused by one-time pumping and descending of a traditional mixed well, a shallow well with a filter screen located in a phreatic layer and a deep well extending to a confined aquifer are arranged in a foundation pit in the construction method; measuring and setting out, exploring and digging, hole forming, hole cleaning and slurry changing, well pipe descending, filling, water stopping and well washing are sequentially completed, and then an independent pumping and descending system is formed; water level, water quantity and peripheral deformation data are obtained through a single well-group well test, and a deep well starting threshold value is determined; before the foundation pit is excavated, only a shallow well is started to drain a phreatic bed, deep well pressure reduction is started in an echelon mode after the foundation pit is excavated to the preset depth and a support is erected, and on-demand dewatering is achieved; the bottom plate construction stage is divided into three stages for well shut-in. The construction method can obviously reduce the drop amplitude of the water level outside the pit, the lateral displacement of the wall body and the ground surface settlement, has the advantages of safety, economy and environmental protection, and is suitable for coastal complex geology deep foundation pit engineering.

Description

Technical Field

[0001] The present application relates to the technical field of dewatering construction methods, and in particular to a dewatering construction method for deep and shallow wells in foundation pits connecting multiple aquifers. Background Art

[0002] In coastal areas, multiple aquifers are often found within the soil, necessitating dewatering during foundation pit construction. If hydraulic connections exist between these aquifers, this can cause significant fluctuations in the water level outside the pit, potentially leading to a range of environmental issues.

[0003] The traditional approach is to place a "mixing well" within the foundation pit, with a filter mesh running through the submerged layer and the pressure-bearing layer, to achieve both dewatering and pressure reduction through a single pumping operation. However, once the mixing well is activated, regardless of the excavation depth, both the submerged layer and the pressure-bearing layer are simultaneously pumped down, causing a significant drop in the water level outside the pit and triggering surrounding surface subsidence. This also reduces the pressure differential between the two holes in the ground-connected wall, increasing wall deformation. If the water-stop curtain fails to completely isolate the pressure-bearing water, the risk of sudden inrush still exists. Summary of the Invention

[0004] Based on this, an embodiment of the present application provides a method for dewatering deep and shallow wells in foundation pits connecting multiple aquifers, which is used to solve the problem of dewatering deep foundation pits due to the large burial depth of pressurized water, the difficulty of the water-stop curtain in completely cutting off the pressurized water, the complex geological conditions of the soil layer, and the presence of many lenses.

[0005] The present application provides a method for dewatering foundation pits with multiple aquifers connected to each other through shallow and deep wells. The method comprises:

[0006] According to the geological and hydrogeological conditions of the foundation pit, shallow wells and deep wells of different depths are arranged in the foundation pit; the shallow well filter is located in the phreatic aquifer, and the deep well filter extends to the confined aquifer;

[0007] Complete surveying and setting out, hole location determination, exploration and excavation, hole completion, hole cleaning and slurry replacement, well pipe lowering, filling, water stopping and well washing in sequence to form a deep and shallow well system with independent pumping and lowering capabilities;

[0008] Conducting a single well test and then a group well test on the deep and shallow well system to obtain water level, water output and surrounding deformation data, and determining the deep well start-up water level threshold based on the data;

[0009] Before excavation, only shallow wells are used to drain the submerged layer. When the excavation depth reaches the preset value and the support system is completed, deep wells are activated in stages to depressurize the confined aquifer, achieving on-demand dewatering.

[0010] During the base plate construction process, the dewatering wells that are no longer needed will be sealed in stages until the foundation pit project is completed.

[0011] Optionally, the steps of measuring, laying out and determining hole positions include:

[0012] Use a total station to mark out the well location according to the designed coordinates. After the drilling rig is in place, use a spirit level to calibrate it so that the center of the hole mouth, the center of the turntable, and the large hook are on the same plumb line. Before drilling, remeasure the outer diameter of the drill bit to ensure that the hole diameter is met.

[0013] Optionally, the excavation step includes:

[0014] After the general contractor completes the underground pipeline briefing, they will first scan with an ultrasonic detector, then use manual digging or a small excavator to explore point by point. If water seepage is encountered, immediately use a 7.5kW sewage pump to pump out the water. Drilling can only be carried out after confirming that there are no pipelines.

[0015] Optionally, the hole forming, hole cleaning and slurry replacement steps include:

[0016] A positive circulation rotary drill with a three-wing drill bit was used. Light pressure and slow rotation were applied in the plain fill section. Appropriate pressure and speed were applied after entering the silt and sand layers. Self-made mud was used in the hole during drilling, and the mud density was controlled at 1.05–1.20 g / cm 3 , when lifting the drill or stopping work, keep the hole full of slurry to prevent hole collapse;

[0017] After drilling to the designed depth, lift the drill bit 0.5m from the bottom of the hole and pump in new slurry to punch the hole until the returned slurry does not contain mud blocks and the specific gravity is less than the preset value.

[0018] Optionally, the step of lowering the well pipe includes:

[0019] The well pipes are pre-arranged according to the designed well depth. The steel pipe well uses a bridge-type water filter pipe and is wrapped with two layers of 60-mesh filter screen. When lowering the pipe, it should be kept vertical and fall naturally into place. The bottom of the sandless cement pipe is equipped with a wooden bottom well tray and a steel wire rope and slowly lowered, and the internodes are fixed with bamboo strips and iron wire.

[0020] Optionally, the filling step comprises:

[0021] Use 0.2-0.3cm medium-coarse sand and continuously and evenly feed it along one side of the wellhead at a feeding speed of ≤0.1m 3 / min, measure the top surface of the filter material at no less than 4 points for every 1m of filling, start the sewage pump in the well to pump water, and inject clean water outside the well for reverse circulation to ensure that the final elevation of the sand surface meets the design requirements.

[0022] Optionally, the water stopping step includes:

[0023] After the gravel is added and allowed to stand for 20 minutes, high-quality clay balls are slowly added to stop the water after the gravel is settled. The water-stopping clay balls are made of high-quality clay, and the water-stopping height is carried out according to the design requirements. After the clay balls are added to the designed height, the clay can be backfilled to the ground height.

[0024] Optionally, the well washing step includes:

[0025] First, use an air compressor to wash the well, and then use a water pump to wash the well and remove the sand at the bottom of the well; during the well washing process, ensure that water flows continuously during the test pumping.

[0026] Optionally, sealing the dewatering well includes:

[0027] In the first stage, the spare wells are sealed with concrete after pipe cutting before pouring the bottom plate cushion layer; in the second stage, after the bottom plate reaches the required strength, the well filter pipes are replaced with flat pipes and water-stop steel rings are welded, and then backfilled with micro-expansive concrete; in the third stage, after the design confirms that the anti-floating and post-casting strips meet the requirements, the last batch of wells are sealed with compaction grouting, and the observation wells outside the pit are backfilled with clay and sealed with steel plates.

[0028] Optionally, a single well test is first performed on the deep and shallow well system, followed by a group well test, specifically including:

[0029] The initial water level of each well was recorded before the test. After the water level stabilized, a single-well pumping test was carried out for 24 hours. The water level changes of the drainage wells around the pumping well and the observation wells outside the pit were observed simultaneously. After the pump was stopped, the observation was continued for 24 hours to obtain the return water data. Subsequently, a group well pumping test was carried out. The group well dewatering cycle lasted for 6 days. A drainage well was set up in the pit as the pit drainage observation well. The water level changes of the observation wells inside the pit and outside the pit were observed simultaneously. During the test, the water pumps of the pumping wells were placed in stages. In the first stage, the water pumps were placed 15 meters below the ground. In the second stage, the water pumps were placed at the bottom of the well. Before the start of the test, water meters were installed in all pumping wells to record the water output.

[0030] The beneficial effects of the technical solutions provided in the embodiments of the present application include:

[0031] (1) Through the "deep and shallow wells + staged opening" strategy, deep wells are only opened when necessary, significantly reducing disturbance to the confined aquifer and the drop in water levels outside the pit. Finite element analysis and field tests have shown that this method can simultaneously reduce lateral displacement of the ground-connected wall and surface settlement outside the pit, meeting the protection requirements of adjacent buildings.

[0032] (2) Drain the submerged layer in advance to improve the strength of the excavated soil. The dynamic monitoring and adjustment mechanism can prevent sudden surge risks in real time. This reduces over-pumping, reduces pumping volume and recharge requirements, shortens the rainfall cycle, and saves electricity and water resources. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] To more clearly illustrate the embodiments of this application or the technical solutions in the prior art, the following briefly introduces the drawings required for the embodiments or the description of the prior art. Obviously, the drawings described below are merely illustrative, and those skilled in the art can derive other implementation drawings based on the provided drawings without inventive effort.

[0034] Figure 1 A construction process flow chart of a method for dewatering a foundation pit with deep and shallow wells connecting multiple aquifers provided in an embodiment of the present application;

[0035] Figure 2 A schematic diagram of the air compressor well washing principle provided in an embodiment of the present application;

[0036] Figure 3 A schematic diagram of a well sealing method provided in an embodiment of the present application. DETAILED DESCRIPTION

[0037] In order to make the purpose, technical solutions and advantages of this application more clear, the following further describes this application in detail with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit this application.

[0038] In the description of the present application, the terms "including", "having" and any variations thereof are intended to cover non-exclusive inclusions. For example, a process, method, system, product or apparatus comprising a series of steps or units is not necessarily limited to those steps or units that have been explicitly listed, but may also include other steps or units that are not explicitly listed but are inherent to these processes, methods, products or apparatuses, or steps or units that are added based on further optimization schemes conceived in the present application.

[0039] In my country's coastal areas, multiple aquifers are often distributed in the soil layer, and dewatering is required during foundation pit construction. If there is hydraulic connection between different aquifers, the dewatering process will cause large water level fluctuations outside the pit, triggering a series of environmental problems. Based on the dewatering test results of a subway line foundation pit project, the finite element software Abaqus was used to study the effect of the presence of silt lenses in the soil layer at the toe of the diaphragm wall on the deformation of the foundation pit. The results show that the presence of lenses in the weak permeable layer at the toe of the diaphragm wall increases permeability, which will increase the hydraulic connection between the inside and outside of the foundation pit, reduce the pore pressure difference and deformation of the diaphragm wall on both sides, but increase settlement outside the pit. It is recommended to use "deep and shallow wells" to dewater multiple aquifers in a tiered manner to better achieve on-demand dewatering and control foundation pit deformation.

[0040] A subway line section was constructed using the open-cut method. There are many pipelines around the structure. The groundwater in the foundation pit is mainly groundwater in the shallow soil layer, and the main sources of recharge are atmospheric precipitation and surface water. The groundwater level changes due to the influence of rainfall and surface water. The groundwater aquifer is mainly the marine layer of the middle Holocene formation (Q4 2 m) and the soil layer above it, which is locally pressure-bearing and mainly composed of silt, sand and artificial fill on the surface, with the lake facies (Q4 1 h) Sedimentary silty clay and riverbed to floodplain phase (Q4 1a1) The sedimentary layer serves as a relatively impermeable base. This aquifer is primarily composed of silt, which exhibits stratification and is interbedded with silty clay in some areas, with intercalated silt lenses in other areas. The retaining structure isolates the first confined aquifer but not the second, posing a significant construction risk.

[0041] The characteristics of this application method include:

[0042] The submerged aquifer excavated during the project is primarily composed of miscellaneous fill, clay, silty clay, and clayey silt. The finer particles in the submerged aquifer make it difficult for construction machinery to operate on the excavation surface during excavation. To address the risk characteristics of the upper submerged aquifer, dewatering wells were deployed within the foundation pit. Pre-excavation was conducted for a period of time to reduce the soil's moisture content. Clay and silty soil layers with poor permeability are difficult to dewater during excavation. This was achieved through the use of open drainage measures, ensuring effective dewatering and facilitating both excavation and normal construction on the excavation surface.

[0043] To meet the needs of foundation pit dewatering and precipitation, the bottom of the precipitation well filter usually needs to be located 5 to 6 meters below the excavation surface. As the depth of the foundation pit increases, a confined aquifer will exist within 5 to 6 meters below the excavation surface. In order to meet the dewatering requirements or anti-surge requirements, this confined layer also needs to be dewatered to a certain extent. At this time, the precipitation well needs to enter this confined aquifer. In this case, two precipitation schemes can generally be adopted. One is a "mixed well" scheme that uses a filter to penetrate the shallow layer and the confined layer. The other is to arrange separate precipitation wells in the shallow layer and the confined layer respectively, forming a "deep-shallow well" scheme.

[0044] When pumping water from a mixed well, all soil layers penetrated by the filter will be affected by the water, regardless of the depth of the water drop. For deep-shallow wells, when water is being pumped from the shallow layer, only the shallow well can be opened to pump water from that layer, leaving the pressure-bearing layer below the pit bottom unaffected. Once excavation reaches a certain depth, the deep well can be opened to pump water from that layer, achieving a "water-on-demand" approach. This solution can significantly reduce deformation caused by water drop.

[0045] The drainage wells in the pit are opened in stages using "deep and shallow wells" to achieve precipitation as needed.

[0046] This method is suitable for coastal cities in my country. Due to the great depth of buried confined water, sometimes it is difficult for the water-stop curtain to completely cut off the confined water. The geological conditions of the soil layer are complex, and there are many lenses in the deep foundation pit dewatering construction.

[0047] This application specifically discloses a dewatering construction method for a foundation pit with multiple aquifers connected to the ground, and the construction method includes:

[0048] According to the geological and hydrogeological conditions of the foundation pit, shallow wells and deep wells of different depths are arranged in the foundation pit; the shallow well filter is located in the phreatic aquifer, and the deep well filter extends to the confined aquifer;

[0049] Complete surveying and setting out, hole location determination, exploration and excavation, hole completion, hole cleaning and slurry replacement, well pipe lowering, filling, water stopping and well washing in sequence to form a deep and shallow well system with independent pumping and lowering capabilities;

[0050] Conducting a single well test and then a group well test on the deep and shallow well system to obtain water level, water output and surrounding deformation data, and determining the deep well start-up water level threshold based on the data;

[0051] Before excavation, only shallow wells are used to drain the submerged layer. When the excavation depth reaches the preset value and the support system is completed, deep wells are activated in stages to depressurize the confined aquifer, achieving on-demand dewatering.

[0052] During the base plate construction process, the dewatering wells that are no longer needed will be sealed in stages until the foundation pit project is completed.

[0053] The principles of this process include:

[0054] Dewatering wells and observation wells outside the pit are constructed, and "deep and shallow wells" are used for drainage and dewatering in the pit. According to the situation of the retaining structure separating the pressurized water layer, it is planned to use a 21-meter dewatering well as a "shallow well" and a 25-meter dewatering well as a "deep well". No separate pressure relief wells are set up, and dewatering is carried out in stages. After the excavation surface reaches two layers of support and the installation is completed, the "deep well" is started to reduce the impact of dewatering on the deformation of the retaining structure and the surrounding environment of the foundation pit. Before dewatering, a dewatering test is conducted to verify the dewatering effect and amount.

[0055] Before the test, the initial water level of each well was recorded. Once the water level stabilized, the test began. First, a single well was dewatered, and water level changes in surrounding drainage wells and observation wells outside the pit were simultaneously observed. The single-well test lasted 24 hours, followed by a 24-hour stop to observe the return water in the drainage wells. A group well pumping test was then conducted, with a six-day dewatering cycle. One drainage well was set up in the pit as an observation well for the dewatering process. Simultaneous observations were made in both the observation wells inside and outside the pit. The pumps for the pumping wells were placed in three steps: the first step was to 15 meters below ground level, and the second step was to place the pumps at the bottom of the well. A water meter was installed in each pumping well before the test began; the third step was to observe the water level after the water level had recovered.

[0056] Based on the test data, a finite element model was established using ABAQUS. The calculation took fluid-solid coupling into consideration, and the water level changes of the aquifers inside and outside the foundation pit were analyzed. The development of surface settlement outside the foundation pit was analyzed, and the lateral displacement of the ground-connected wall was analyzed.

[0057] Please refer to Figure 1 , which shows a flowchart of a dewatering construction method for a foundation pit with multiple aquifers connected to each other and shallow wells provided by an embodiment of the present application. Specific construction operation points may include the following:

[0058] 1. Measure and lay out, and determine the hole position:

[0059] The drilling rig should be positioned securely and horizontally, with the hole center, grinding disc center, and hook aligned perpendicularly. Drilling can only begin after the wellbore and sand material are in place. The entire hole must be round and smooth, and bent drill rods are not permitted. Before drilling, confirm the drill bit size to ensure it meets the required hole diameter.

[0060] 2. Exploration:

[0061] (1) Before excavation, the general reporting unit must have a clear understanding of the underground pipelines within the excavation range and use an ultrasonic detector to detect the pipelines within the excavation range. After the general contractor has completed the briefing, the excavation work will begin.

[0062] (2) Professional protective personnel shall conduct manual drilling and exploration, or an excavator shall be used to dig holes directly.

[0063] (3) When underground water seepage occurs during drilling, a 7.5KW sewage pump is used to drain the water.

[0064] 3. Hole formation:

[0065] (1) Drilling a hole.

[0066] The engineering drilling equipment uses a positive circulation drilling rig, and the diameter is selected according to the design and specification requirements. According to construction experience, the use of a three-wing drill bit has good construction stability, can ensure the quality of the hole, and can effectively control the shrinkage phenomenon in the hole, laying the foundation for ensuring the quality of the project.

[0067] When drilling, apply light pressure and rotate slowly. When the drill bit has completely entered the silt layer and sand layer, you can apply appropriate pressure and increase the rotation speed.

[0068] The hole construction adopts natural slurrying in the hole. The mud density is controlled at 1.05-1.20 during the drilling process. When the drill tool is lifted or work is stopped, the hole must be filled with mud to prevent the hole wall from collapsing.

[0069] (2) Clean the hole and change the slurry.

[0070] After the hole is drilled to the designed elevation, the drill rod is lifted to 0.50m from the bottom of the hole before the drill is lifted, and the hole is punched to remove debris. At the same time, the mud density in the hole is gradually lowered until the returned mud does not contain mud lumps.

[0071] 4. Lowering pipe:

[0072] Steel Pipe: Arrange and assemble the well pipes in advance according to the designed well depth. When lowering the pipes, strictly control the bottom elevation of all deep wells and maintain a consistent wellhead elevation. The well pipes should be lowered smoothly into the hole, and both ends of each section should be leveled. The well pipes should be wrapped with two layers of 60-mesh filter screen. The pipes should be lowered precisely into position. Allow them to fall naturally, gently rotating them into place. Do not forcefully press them down to avoid damaging the filter structure. Steel well pipes should be installed using the lifting and lowering method. Steel well pipes can be connected using butt welding or threaded connections; flange connections are not recommended.

[0073] Sandless cement pipe: Before lowering the sandless cement pipe into the well, prepare a steel wire rope (about Φ5mm), fix one end of the steel wire rope, tie a section of sandless cement pipe and a wooden well bottom together, pass the steel wire rope through the groove in the center of the well bottom, and then slowly put it into the hole. Use bamboo strips and iron wire to fix the well pipes, and slowly release the steel wire rope until the well pipe is lowered.

[0074] 5. Filler:

[0075] In this embodiment, the dewatering well should use medium-coarse sand with a particle size of 0.2 to 0.3 cm, without powder. The specific operating requirements of the filler are as follows:

[0076] It should be fixed along the wellhead, rotated in a single direction, and continuously and evenly distributed step by step. The distribution speed should not exceed 0.1m3 / min.

[0077] It is recommended to use a shovel or similar sized object to add materials. It is strictly forbidden to use a cart to dump materials into the well. It is strictly forbidden to shake the well pipe during the feeding process.

[0078] The top elevation of the filter material in the well should be measured in time, with at least 4 measuring points evenly distributed along the circumference of the wellhead, and the lowest point elevation shall be used as the standard;

[0079] When the elevation of the top surface of the filter material reaches the design requirements, the actual volume of filter material invested should not be less than the theoretical volume.

[0080] In addition, to ensure that the filler is in place, a sewage pump should be placed in the well. After the filler is in place, start the sewage pump inside the well to pump water, and at the same time, inject clean water outside the pit. In this way, driven by the water flow inside and outside the well, the analytical particles in the sand material will be carried away by the water flow, which can reduce the mud concentration and prevent mud skin from wrapping, ensuring the water output effect of the precipitation well. After the above operations are completed, measure the filler height again to confirm whether the filler is in place.

[0081] 6. Waterstop:

[0082] After gravel placement, stop the pump and wait for 20 minutes. Once the gravel has settled, slowly add high-quality clay balls to seal the water. These water-stopping clay balls are made of high-quality clay, and the water-stop height is determined according to the design. Once the clay balls have reached the designed height, backfill with clay to the ground level.

[0083] 7. Well washing:

[0084] First, use an air compressor to flush the well, and finally use a water pump to flush and remove any sand from the bottom. Dewatering wells should be cleaned promptly after filling is completed to avoid mud settling, which can lead to a thick, hardened mud crust on the well walls and severely impact water filtration and output. Since the filter pipes for relief wells are relatively short, flushing should be performed after clay backfill is completed around the relief well. Ensure continuous flow during the pumping test. Air compressor well flushing uses a complete set of equipment. A special flushing nozzle is used to pump air into the well bottom, stirring the mud and sediment. This creates a negative pressure at the wellhead, allowing atmospheric pressure to blow the mixed mud and sediment out. This process continues for a period of time, as water continues to flow into the well, discharging the mud and sediment until the water is clear and sand-free.

[0085] 8. Seal the well:

[0086] The well sealing is divided into three main stages: sealing the well before pouring the bottom plate (that is, sealing the well when pouring the concrete cushion layer), sealing the well after the construction of the structural bottom plate and side wall, and sealing the well after the design confirms that the anti-floating and post-pouring strips meet the requirements.

[0087] Phase 1: Before base slab construction, the foundation pit is excavated to the base. During the base slab cushion construction, some dewatering wells not in operation (spare and observation wells) with low water output can be sealed first. Before sealing, other working wells in the foundation pit continue to operate, controlling the water level to always be below 1.0m below the excavation surface. The number of wells sealed during this phase is generally no more than 1 / 3 of the total number of dewatering wells in the foundation pit, predicated on preventing sudden inrush during the base slab pouring.

[0088] Phase II: After the baseplate is poured and reaches its designed strength, some wells are reserved for emergency, structural anti-floating, and post-casting wells, while the remaining wells are sealed. The number of wells sealed during this phase will be determined based on the specific pumping conditions on site.

[0089] Phase 3: After the architectural design unit confirms that the anti-floating and post-pouring strips meet the requirements, all reserved drainage wells can be sealed.

[0090] The specific number of wells that can be sealed at each of the above stages will be determined by the design firm based on the actual on-site dewatering well operation and water level control. The implementation of well sealing requires the consent of the design firm and the general contractor.

[0091] The first stage is to seal the well generally. When the foundation pit is excavated to the base, the well pipe above the base is removed and concrete is poured at the well pipe position during the construction of the base concrete cushion. Figure 2 shown.

[0092] The drainage wells in the second and third stages need to pass through the bottom plate. After the concrete pouring of the structural bottom plate is completed and the structure meets the anti-floating requirements, the wells will be sealed. Therefore, the drainage well pipes need to be processed before the bottom plate construction. Since the drainage well pipes are bridge-type filter pipes, in order to ensure a tight connection between the bottom plate and the precipitation well pipe when the well pipes pass through the bottom plate and prevent leakage, the original water filter pipes need to be replaced with flat pipes from above the base, and a rigid water-stop steel ring (width not less than 15cm) is welded at the position of the flat pipe corresponding to the middle of the bottom plate to ensure that groundwater will not leak along the interface between the foundation pit bottom plate and the precipitation well pipe.

[0093] The second and third stages of well sealing are to seal the well after the bottom plate construction is completed. The specific operation methods are as follows:

[0094] Reserve a drainage well and replace the flat pipe within the base plate. Clean the soil around the well pipe before pouring the base plate. Use sandpaper to polish the steel pipe on the outside of the pipe wall to remove rust, and ensure that the contact surface between the well pipe and the concrete is dense when the base plate concrete is poured later.

[0095] After the base plate reaches the designed strength and meets the requirements for well sealing, high-quality clay (or plain cement) is backfilled into the well pipe to 2m below the base. The remaining portion of the well pipe to 20cm below the top of the base plate is backfilled with micro-expansive concrete.

[0096] After the backfill concrete reaches the designed strength, remove the remaining water in the well pipe, cut off the remaining unbackfilled well pipe, and weld a 1cm thick steel plate at the wellhead to seal it.

[0097] After the well is sealed, use micro-expansive concrete with a grade one higher than the structural bottom plate to backfill the well pipe until it is flush with the top surface of the top plate.

[0098] In the third stage, the well is sealed. The water output of the reserved dewatering well in the pit is large, so compaction grouting can be considered to seal the well.

[0099] Basically, after the precipitation in this embodiment is completely completed, the water level observation well outside the pit is backfilled with clay to the pipe mouth. The steel pipe well is finally sealed with a steel plate and welded firmly to complete the sealing of the well outside the pit.

[0100] According to the design requirements, bored pile position measurement control points were laid out and numbered. After verification, the pile center points were measured and placed on-site, and guard piles were placed around the pile positions. Manually excavated holes were spaced 1.5 meters apart, with a diameter of 1 meter and a total depth of 10 meters. A construction plan of excavating every 4 holes was adopted. Before construction, technicians and construction workers thoroughly inspected all construction preparations hole by hole, conducting technical and safety briefings at each level to ensure that all safety and technical measures were implemented ideologically, organizationally, and operationally.

[0101] Table 1 shows the main material usage plan, and Table 2 shows the main mechanical equipment configuration plan.

[0102] Table 1 Main material usage plan

[0103]

[0104] Table 2 Main mechanical equipment configuration

[0105] Material name Specification quantity unit Remark precipitation well pipe Φ219 several m steel pipe Dewatering well filter pipe Φ219 several m Bridge filter, bridge height 1.0mm Sedimentation tube Φ219 several m Sandless cement pipe Φ400 several m filter media Medium coarse sand several <![CDATA[m 3 ]]> water-stop clay clay ball several t

[0106] The test methods of this application include:

[0107] 1. Single well test.

[0108] A deep dewatering well within the foundation pit was selected as a test well. A water pump was placed at the bottom of the well. Water level changes in a nearby dewatering well within the pit and an observation well outside the pit were simultaneously monitored, and the water output was recorded. Pumping lasted for one day. The water level recovered within one day.

[0109] 2. Group well test.

[0110] Step 1: Place the pump for the dewatering well within the foundation pit at a distance of 15 meters. Select the dewatering well within the foundation pit as the pumping well, and reserve one dewatering well as a water level observation well within the pit. Simultaneously monitor water level changes in the observation wells outside the pit and record the water output from each pumping well. Pumping should continue for three days.

[0111] Step 2: Place the pump at the bottom of the dewatering well within the foundation pit. Select a deep dewatering well within the foundation pit as the pumping well, and reserve one shallow dewatering well as a water level observation well within the pit. Simultaneously monitor water level changes in the observation wells outside the pit and record the water output from each pumping well. Pumping will continue for two days.

[0112] Step 3: Organize the test data, analyze the test results, water volume analysis, surface subsidence analysis outside the pit, and lateral displacement analysis of the ground-anchored wall.

[0113] The effects of tiered dewatering of deep and shallow drainage wells include:

[0114] 1. Initial water level collection.

[0115] Specifically, Table 3 gives the initial water level statistics.

[0116] Table 3 Initial water level statistics

[0117] Serial number hashtag Initial water level depth (m) date 1 S-1 8.42 2022 / 6 / 1 2 S-2 8.11 2022 / 6 / 1 3 S-3 8.13 2022 / 6 / 1 4 S-4 8.16 2022 / 6 / 1 5 S-5 8.08 2022 / 6 / 1 6 S-6 8.01 2022 / 6 / 1 7 S-7 8.11 2022 / 6 / 1 8 S-8 7.97 2022 / 6 / 1 9 S-9 7.35 2022 / 6 / 1 10 S-10 8.17 2022 / 6 / 1 11 S-11 7.98 2022 / 6 / 1 12 S-12 8.13 2022 / 6 / 1 13 S-13 8.18 2022 / 6 / 1 14 S-14 8.10 2022 / 6 / 1 15 S-15 8.13 2022 / 6 / 1 16 S-16 7.94 2022 / 6 / 1 17 S-17 8.06 2022 / 6 / 1

[0118] The initial values ​​of the S1-S17 dewatering wells were collected on June 1, 2022, and the test wells were initially digitized before the pumps were started.

[0119] 2. Analysis of pumping test data.

[0120] The test was divided into three phases: single well dewatering, group well dewatering, and pump stop and water return. Table 4 shows the statistical table of water output data for the group wells.

[0121] Table 4 Statistics of water output of group wells

[0122]

[0123] From the above we can see that:

[0124] (1) The weak permeable layer at the bottom of the foundation pit dewatering well and the toe of the diaphragm wall contains silt lenses, which increases permeability, increases the dewatering depth and the hydraulic connection between the inside and outside of the foundation pit, and increases the deformation of the diaphragm wall and the settlement outside the pit. The location where the pore pressure changes the most inside the pit is located in the pressure-bearing layer where the dewatering well filter penetrates, while the location where the pore pressure changes the most outside the foundation pit is located in the soil layer where the toe of the diaphragm wall is located. Under complex geological conditions, if the hydraulic connection between the various aquifers is unclear, the water level changes and the order of changes in the pressure-bearing layer near the toe of the outer wall of the foundation pit should be observed in time during the dewatering of the foundation pit project to understand the hydraulic connection between the inside and outside of the pit and prevent a large drop in the water level outside the pit.

[0125] (2) The weak permeable layer at the toe of the ground-connected wall contains lenses, which increase permeability. This will increase the hydraulic connection between the inside and outside of the foundation pit, reduce the pore pressure difference on both sides of the ground-connected wall, and thus reduce the deformation of the ground-connected wall. However, it will also cause the water level outside the foundation pit to drop, increasing the settlement outside the pit. The maximum settlement outside the pit is not located at the surface, but in the shallow soil at a certain depth. Below the maximum settlement position, the soil pore pressure drops significantly, while above this position, the pore pressure remains almost unchanged.

[0126] (3) When there is an aquifer near the bottom of the foundation pit, the foundation pit dewatering should adopt "deep-shallow wells" to better achieve on-demand dewatering and foundation pit deformation control. Calculations show that when only the shallow well is opened for dewatering in the "deep-shallow well" method, the water level of the pressure-bearing layer below the bottom of the pit is almost unaffected, and the deformation of the ground-connected wall and the surface settlement outside the pit are both less than those in the "mixed well" method. After the deep well is opened, the drainage effect in the foundation pit is the same as that of the "mixed well", but because the support has been erected when the deep well is opened, the deformation of the ground-connected wall and the settlement outside the pit in the "deep-shallow well" method are significantly less than those in the "mixed well" method. After the subsequent optimization of similar construction plans in this application, economic and social benefits have been successfully achieved, proving the reliability of this method.

[0127] The technical features of the above-mentioned embodiments can be combined arbitrarily. In order to make the description concise, not all possible combinations of the technical features in the above-mentioned embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0128] The above-described embodiments merely represent several implementation methods of the present application. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent application. It should be noted that a person of ordinary skill in the art may make various modifications and improvements without departing from the spirit of the present application, and these modifications and improvements fall within the scope of protection of the present application. Therefore, the scope of protection of the present patent application shall be determined by the appended claims.

Claims

1. A method for dewatering foundation pits with deep and shallow wells in multiple aquifer-connected strata, characterized in that: The construction method includes: According to the geological and hydrogeological conditions of the foundation pit, shallow wells and deep wells of different depths are arranged in the foundation pit; the shallow well filter is located in the phreatic aquifer, and the deep well filter extends to the confined aquifer; Complete surveying and setting out, hole location determination, exploration and excavation, hole completion, hole cleaning and slurry replacement, well pipe lowering, filling, water stopping and well washing in sequence to form a deep and shallow well system with independent pumping and lowering capabilities; Conducting a single well test and then a group well test on the deep and shallow well system to obtain water level, water output and surrounding deformation data, and determining the deep well start-up water level threshold based on the data; Before excavation, only shallow wells are used to drain the submerged layer. When the excavation depth reaches the preset value and the support system is completed, deep wells are activated in stages to depressurize the confined aquifer, achieving on-demand dewatering. During the base plate construction process, the dewatering wells that are no longer needed will be sealed in stages until the foundation pit project is completed.

2. The construction method according to claim 1, characterized in that: The steps of measuring, laying out and determining the hole positions include: Use a total station to mark out the well location according to the designed coordinates. After the drilling rig is in place, use a spirit level to calibrate it so that the center of the hole mouth, the center of the turntable, and the large hook are on the same plumb line. Before drilling, remeasure the outer diameter of the drill bit to ensure that the hole diameter is met.

3. The construction method according to claim 1, characterized in that: The excavation step includes: After the general contractor completes the underground pipeline briefing, they will first scan with an ultrasonic detector, then use manual digging or a small excavator to explore point by point. If water seepage is encountered, immediately use a 7.5kW sewage pump to pump out the water. Drilling can only be carried out after confirming that there are no pipelines.

4. The construction method according to claim 1, characterized in that: The hole forming, hole cleaning and slurry changing steps include: A positive circulation rotary drill with a three-wing drill bit is used. Light pressure is applied and slow rotation is applied in the plain fill section. Appropriate pressure and speed are applied after entering the silt and sand layers. In-hole slurry is generated during drilling, with a controlled slurry density of 1.05–1.20 g / cm³. When the drill is lifted or work is stopped, the hole is kept full of slurry to prevent collapse. After drilling to the designed depth, lift the drill bit 0.5m from the bottom of the hole and pump in new slurry to punch the hole until the returned slurry does not contain mud blocks and the specific gravity is less than the preset value.

5. The construction method according to claim 1, characterized in that: The step of lowering the pipe into the well comprises: The well pipes are pre-arranged according to the designed well depth. The steel pipe well uses a bridge-type water filter pipe and is wrapped with two layers of 60-mesh filter screen. When lowering the pipe, it should be kept vertical and fall naturally into place. The bottom of the sandless cement pipe is equipped with a wooden bottom well tray and a steel wire rope and slowly lowered, and the internodes are fixed with bamboo strips and iron wire.

6. The construction method according to claim 1, characterized in that: The packing step comprises: Use 0.2-0.3cm medium-coarse sand and continuously and evenly feed it along one side of the wellhead at a feeding speed of ≤0.1m 3 / min, measure the top surface of the filter material at no less than 4 points for every 1m of filling, start the sewage pump in the well to pump water, and inject clean water outside the well for reverse circulation to ensure that the final elevation of the sand surface meets the design requirements.

7. The construction method according to claim 1, characterized in that: The water stopping step comprises: After the gravel is added and allowed to stand for 20 minutes, high-quality clay balls are slowly added to stop the water after the gravel is settled. The water-stopping clay balls are made of high-quality clay, and the water-stopping height is carried out according to the design requirements. After the clay balls are added to the designed height, the clay can be backfilled to the ground height.

8. The construction method according to claim 1, characterized in that: The well washing step comprises: First, use an air compressor to wash the well, and then use a water pump to wash the well and remove the sand at the bottom of the well; during the well washing process, ensure that water flows continuously during the test pumping.

9. The construction method according to claim 1, characterized in that: Sealing a dewatering well includes: In the first stage, the spare wells are sealed with concrete after pipe cutting before pouring the bottom plate cushion layer; in the second stage, after the bottom plate reaches the required strength, the well filter pipes are replaced with flat pipes and water-stop steel rings are welded, and then backfilled with micro-expansive concrete; in the third stage, after the design confirms that the anti-floating and post-casting strips meet the requirements, the last batch of wells are sealed with compaction grouting, and the observation wells outside the pit are backfilled with clay and sealed with steel plates.

10. The construction method according to claim 1, characterized in that: The deep and shallow well system is first tested on a single well and then on a group of wells, specifically including: The initial water level of each well was recorded before the test. After the water level stabilized, a single-well pumping test was carried out for 24 hours. The water level changes of the drainage wells around the pumping well and the observation wells outside the pit were observed simultaneously. After the pump was stopped, the observation was continued for 24 hours to obtain the return water data. Subsequently, a group well pumping test was carried out. The group well dewatering cycle lasted for 6 days. A drainage well was set up in the pit as the pit drainage observation well. The water level changes of the observation wells inside the pit and outside the pit were observed simultaneously. During the test, the water pumps of the pumping wells were placed in stages. In the first stage, the water pumps were placed 15 meters below the ground. In the second stage, the water pumps were placed at the bottom of the well. Before the start of the test, water meters were installed in all pumping wells to record the water output.

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